This invention relates to switching to connect a load to a power source, with isolation from the control point, and suppression of the inrushing surge of current when the switch is initially closed.
The requirement for isolation of the control source from a power switching arrangement has been handled in the prior art in various ways. In some situations pulse transformers may be used. However, for low frequency response the transformers become too large. It is also well known to use light coupling devices. For example U.S. Pat. No. 4,140,914 relates to an isolated signal circuit for separating the input signal from the load by use of a photo transistor. U.S. Pat. No. 4,129,785 discloses a solid state load switching circuit utilizing two light-activated silicon controlled rectifier inputs to minimize transient signal effects in the switch circuits. U.S. Pat. No. 3,421,488 shows a pair of photo transistors for triggering a semiconductor ignition system.
Silicon controlled rectifiers (SCR's) are commonly used for load switching. Many arrangements for transient suppression are also known. U.S. Pat. No. 4,097,770 discloses an SCR switching circuit including an oscillator for producing an initial SCR trigger pulse and a separate triggering circuit activated by a second pulse (closing of the ignition points) for conditioning the SCR to produce a controlled output. U.S. Pat. No. 3,302,629 describes an ignition or load switching circuit having a blocking oscillator charged capacitor and an SCR triggered by a pulse circuit. U.S. Pat. No. 3,622,837 describes a transistorized load switching circuit utilizing an SCR which is triggered by the ignition points of an automobile.